The invention relates to a circuit arrangement for detecting the switching phase of wave trains which periodically exhibit a fixed phase difference relative to a normal condition, in particular the color synchronizing pulses of a color television signal recorded on a video tape recorder, whose polarity changes from field to field, followed by a compensation circuit, in which the pulses, which have been restored to the normal condition are combined, for example added, directly and delayed by the sampling interval, so that in the case of a correct switching rhythm the amplitude value is a maximum.
Such a switching process during the recording of a color television signal, in which the phase of the chrominance signal alternates from line to line every second field, is known from Funkschau 1976, No. 2, pages 61/62. In another recording system two lines are switched sequentially every second field, so that the phase of the relevant field changes from line pair to line pair, while in the intermediate fields the phase remains constant. Other switching systems employ greater groups, of for example four lines each, for the switching rhythm.
When the record carrier is played back the switching process should be cancelled before the signals can be processed further. In the case of a system in which the phase of the chrominance carrier and thus of the color synchronizing signal is inverted every two lines in one field and remains constant in the other field, the signals which have been switched in an opposite sense during playback are applied, directly and with a delay corresponding to two line periods, to an adder stage which supplies the output signal in which crosstalk, which may occur between two adjacent tracks recorded on the tape, is largely eliminated.
In this respect it is essential that the restoration is effected in the correct field rhythm, in conformity with the switching effected during recording, while, for example, the signal of the other field should be transferred without any change.
For the identification of the field that has been subject to switching, an additional signal may be transmitted, but this demands a convention (standardisation) and complicates the compatability of different apparatus.